Development of an equation of state for the square-well chain molecules of variable well width based on a modified coordination number model

An equation of state (EOS) is developed according to the statistical associating fluid theory (SAFT) model. The dispersion term in the new EOS is derived by a modified coordination number equation. This modified coordination number expression gives correct results at low density and close-packed regions. It also agrees well with the computer simulation results for square-well (SW) fluids. A modified expression of the chain term compressibility factor for the SW fluid with variable well width is proposed in this study. Comparison of the calculated compressibility factors with molecular simulation data for tangent hard sphere chains and SW chains indicates that the new EOS yields satisfactory results and relatively smaller deviations than those from other equations. Finally, the new EOS is employed on real fluid systems of n-alkanes. For each pure fluid, four EOS parameters are regressed. These pure fluid parameters yield reasonable correlation with the molecular weight for n-alkanes up to n-eicosane. The pure fluid properties are well calculated by the new EOS. Prediction of the saturated vapor pressures of higher carbon number alkanes also shows satisfactory results. The new EOS developed in this study is consistent with the molecular simulation data for SW fluids with variable well width. It is simple in mathematical form, and is applicable from small molecular weight compounds to long chain molecules.